Antireflection film and optical element having the same

ABSTRACT

An antireflection film includes alternately deposited high-refractive-index layers and low-refractive-index layers. The refractive indexes and extinction coefficients of the layers are such that the antireflection film exhibits sufficient antireflection characteristics even if the geometrical thickness thereof is small. An optical element having the antireflection film on a substrate exhibits high transmittance and has excellent optical characteristics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to antireflection films for coating thesurfaces of optical elements and to optical elements having the same. Inparticular, the present invention relates to an antireflection filmadvantageously used for an ArF excimer laser beam having a wavelength of193 nm.

2. Description of the Related Art

Fluoride films have generally been used as an antireflection film forvacuum ultraviolet light. For example, Japanese Patent Laid-Open Nos.7-244205 and 7-244217 have disclosed optical elements using a fluoridefilm as an antireflection film. It is, however, difficult to reduce thereflectance of the fluoride antireflection film to 0.2% or less in atargeted wavelength band. Also, the fluoride film has an inferiorenvironmental resistance.

Accordingly, oxide films having an environmental resistance superior tothat of the fluoride film, such as Al₂O₃ and SiO₂, are beingincreasingly used. Oxide antireflection films have been disclosed inJapanese Patent Laid-Open Nos. 09-258006 and 2000-206304. Theseantireflection films are composed of high-refractive-index layerscontaining Al₂O₃ and low-refractive-index layers containing SiO₂ thatare alternately layered.

As shown in these publications, each high-refractive-index layer andeach low-refractive-index layer are paired and layered on the opticalelement. The performance of an antireflection film depends on the numberand thickness of the layers. In general, as the number of layers and thenumber of interfaces between the high-refractive-index layers and thelow-refractive-index layers increase, the antireflection characteristicsare enhanced. Also, the above publications disclose that the enhancedantireflection characteristics lead to an optical element having anincreased transmittance. Therefore, antireflection films are generallyformed to at least a specific thickness.

Films always absorb a certain amount of light when they transmit thelight. If the geometrical thickness of the antireflection filmincreases, the absorption also increases, and, consequently, thetransmittance of the resulting optical element is reduced. In general,as the refractive index of a film increases, the film absorbs morelight. Accordingly, absorption is hardly taken into account in thelow-refractive-index layers, such as SiO₂ layers. In thehigh-refractive-index layers, such as Al₂O₃ layers, however, absorptionis not negligible from the viewpoint of optical characteristics. If anantireflection film is produced taking only reflection into account,without considering the absorption of ultraviolet light by the Al₂O₃layers, the transmittance of the resulting antireflection film isreduced. Thus, the resulting element cannot sufficiently function as anoptical element. The foregoing Japanese Patent Laid-Open Nos. 09-258006and 2000-206304 have not described the absorption of ultraviolet lightin the antireflection film.

In general, light absorption is expressed with an extinctioncoefficient. FIG. 12 is a plot showing the relationship between therefractive index and extinction coefficient of an Al₂O₃ layer for lightof 193 nm. The refractive index is changed by varying the amount offluoride gas introduced for forming the film. The horizontal axis andthe vertical axis in FIG. 12 represent the refractive index and theextinction coefficient, respectively. FIG. 12 suggests that if therefractive index of a film is 1.67 or less, the extinction coefficientcan be assumed to be 0. In a film having a refractive index of 1.67 ormore, however, the extinction coefficient is more than 0, and itincreases as the refractive index increases. The extinction coefficientwill be described later in detail.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide anantireflection film in which the extinction coefficient for an ArFexcimer laser beam having a wavelength of 193 nm is controlled to havesufficient antireflection characteristics even if the geometricalthickness thereof is small. Another object of the present invention isto provide an optical element having the antireflection film depositedon the substrate thereof to reduce the geometrical thickness of thefilm, and thus having an increased light transmittance and enhancedoptical characteristics.

According to an aspect of the present invention, an antireflection filmis provided, which includes at least one low-refractive-index layer andat least one high-refractive-index layer. Preferably, more than one ofeach type of layers is provided. The low-refractive-index layers have arefractive index ns satisfying the expression 1.50≦ns≦1.60 and anextinction coefficient ks satisfying the expression 0<ks≦1×10⁻⁵, forultraviolet light having a wavelength of 193 nm. Thehigh-refractive-index layers have a refractive index na satisfying theexpression 1.74≦na≦1.77 and an extinction coefficient ka satisfying theexpression 1×10⁻⁵≦ka≦1×10⁻³, for ultraviolet light having a wavelengthof 193 nm. The total geometrical thickness of the high-refractive-indexlayers is in the range of about 55 to about 65 nm. Also, thelow-refractive-index layers and the high-refractive-index layers arealternately layered.

The high-refractive-index layers may contain Al₂O₃, and thelow-refractive-index layers may contain SiO₂.

According to another aspect of the present invention, an optical elementusing the above-described antireflection film is provided. The opticalelement includes a quartz substrate and an antireflection film composedof six layers formed on the quartz substrate, starting from one of thehigh-refractive index layers. The antireflection film has a designcenter wavelength λ_(o) in the range of 180 nm to 220 nm. The respectiveoptical thicknesses d1, d2, d3, d4, d5, and d6 of the layers at thefirst to sixth positions from the substrate side satisfy the followingexpressions:0.02λ_(o) ≦d 1≦0.10λ_(o),1.00λ_(o) ≦d 2≦2.00λ_(o),0.22λ_(o) ≦d 3≦0.30λ_(o),0.22λ_(o) ≦d 4≦0.30λ_(o),0.22λ_(o) ≦d 5≦0.30λ_(o), and0.22λ_(o) ≦d 6≦0.30λ_(o).

The present invention is also directed to another optical elementincluding a fluorite substrate and an antireflection film composed ofseven layers formed on the fluorite substrate, starting from one of thelow-refractive-index layers. The antireflection film has a design centerwavelength λ_(o) in the range of 180 nm to 220 nm. The opticalthicknesses d1, d2, d3, d4, d5, d6, and d7 of the layers at the first toseventh positions from the substrate side satisfy the followingexpressions:0.00λ_(o) <d 1≦0.15λ_(o),0.02λ_(o) ≦d 2≦0.10λ_(o),1.20λ_(o) ≦d 3≦2.0λ_(o),0.22λ_(o) ≦d 4≦0.30λ_(o),0.22λ_(o) ≦d 5≦0.30λ_(o),0.22λ_(o) ≦d 6≦0.30λ_(o), and0.22λ_(o) ≦d 7≦0.30λ_(o).

According to another aspect of the present invention, an antireflectionfilm is provided, which includes alternately layeredlow-refractive-index layers and high-refractive-index layers. Thelow-refractive-index layers have a refractive index ns satisfying theexpression 1.50≦ns≦1.60 and an extinction coefficient ks satisfying theexpression 0<ks≦1×10⁻⁵, for ultraviolet light having a wavelength of 193nm. The high-refractive-index layers have a refractive index nasatisfying the expression 1.68≦na≦1.74 and an extinction coefficient kasatisfying the expression 1×10⁻⁵≦ka≦1×10⁻³, for ultraviolet light havinga wavelength of 193 nm. The total geometrical thickness of thehigh-refractive-index layers is in the range of 65 to 75 nm.

The high-refractive-index layers may contain Al₂O₃ and thelow-refractive-index layers may contain SiO₂.

According to another aspect of the present invention, an optical elementusing the above-described antireflection film is provided. The opticalelement includes a quartz substrate and an antireflection film composedof eight layers formed on the quartz substrate, starting from one of thehigh-refractive index layers. The antireflection film has a designcenter wavelength λ_(o) in the range of 180 nm to 220 nm. The respectiveoptical thicknesses d1, d2, d3, d4, d5, d6, d7, and d8 of the layers atthe first to eighth positions from the substrate side satisfy thefollowing expressions:0.05λ_(o) ≦d 1≦0.20λ_(o),1.00λ_(o) ≦d 2≦1.50λ_(o),0.05λ_(o) ≦d 3≦0.15λ_(o),0.35λ_(o) ≦d 4≦0.45λ_(o),0.15λ_(o) ≦d 5≦0.25λ_(o),0.27λ_(o) ≦d 6≦0.37λ_(o),0.16λ_(o) ≦d 7≦0.26λ_(o), and 0.23λ_(o) ≦d 8≦0.33λ_(o).

The present invention is also directed to another optical element usingthe foregoing antireflection film. The optical element includes afluorite substrate and an antireflection film composed of nine layersformed on the fluorite substrate, starting from one of thelow-refractive-index layers. The antireflection film has a design centerwavelength λ_(o) in the range of 180 nm to 220 nm. The opticalthicknesses d1, d2, d3, d4, d5, d6, d7, d8, and d9 of the layers at thefirst to ninth positions from the substrate side satisfy the followingexpressions:0.00λ_(o) <d 1≦0.15λ_(o),0.05λ_(o) ≦d 2≦0.20λ_(o),0.04λ_(o) ≦d 3≦1.50λ_(o),0.05λ_(o) ≦d 4≦0.15λ_(o),0.35λ_(o) ≦d 5≦0.45λ_(o),0.15λ_(o) ≦d 6≦0.25λ_(o),0.27λ_(o) ≦d 7≦0.37λ_(o),0.16λ_(o) ≦d 8≦0.26λ_(o), and0.23λ_(o) ≦d 9≦0.33λ_(o).

By controlling the extinction coefficient of each layer of theantireflection film, for an ArF excimer laser beam having a wavelengthof 193 nm, the resulting antireflection film can have sufficientantireflection characteristics even if the geometrical thickness thereofis small.

By depositing the antireflection film having a reduced geometricalthickness on a substrate, an optical element having an increased lighttransmittance and enhanced optical characteristics can be achieved.

The above and other objects of the invention will become more apparentfrom the following description taken in conjunction with theaccompanying drawings.

Further objects, features and advantages of the present invention willbecome apparent from the following description of the preferredembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an optical element according to a firstembodiment of the present invention.

FIG. 2 is a graph showing the spectral reflectance of Example 1.

FIG. 3 is a graph showing the spectral reflectance of Example 2.

FIG. 4 is a sectional view of an optical element according to a secondembodiment of the present invention.

FIG. 5 is a graph showing the spectral reflectance of Example 3.

FIG. 6 is a sectional view of an optical element according to a thirdembodiment of the present invention.

FIG. 7 is a graph showing the spectral reflectance of Example 4.

FIG. 8 is a sectional view of an optical element according to a fourthembodiment of the present invention.

FIG. 9 is a graph showing the spectral reflectance of Example 5.

FIG. 10 is a graph showing the spectral reflectance of ComparativeExample 1.

FIG. 11 is a graph showing the spectral absorption of Example 1 andComparative Example 1.

FIG. 12 is a graph showing the relationship between the refractive indexand the extinction coefficient.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The structural principle of an antireflection film of the presentinvention will now be described. Ultraviolet light emitted to an opticalelement having an antireflection film is divided into the lightreflected from the antireflection film, light transmitted through theantireflection film, and light absorbed by the antireflection film. Thepercentages of the reflection, transmission, and absorption highlydepend on the refractive index of the optical element. In general, therefractive index N of an optical element is expressed by equation 1using a complex number:N=n _(r) −i·k  (1),

-   -   wherein k is an extinction coefficient.

The extinction coefficient refers to the degree of light absorption. Ingeneral, a material having an extinction coefficient k of 1×10⁻³ or lessis used for antireflection films. This is because an antireflection filmwith an extinction coefficient of more than 1×10⁻³ has a significantnegative effect on antireflection characteristics.

First, the description is focused on light reflection in associationwith reflection characteristics of the antireflection film. Reflectionfrom the antireflection film is hardly influenced by the imaginary parti·k of the refractive index N, but is highly influenced by the real partn_(r) when the extinction coefficient k is 1×10⁻³ or less. Hence, theextinction coefficient k has little effect on the antireflectioncharacteristics and does not need to be considered if it is 1×10⁻³ orless.

Next, the description is focused on transmittance, which is one of theoptical characteristics of the optical element having the antireflectionfilm. When the reflectance of an antireflection film is constant, asmall absorptance leads to an increased transmittance of ultravioletlight. Absorption by the antireflection film is influenced by theimaginary part i·k of the refractive index N, as well as the real partn_(r). Accordingly, the extinction coefficient k of the antireflectionfilm must be sufficiently considered. It is generally known that amaterial having a higher refractive index provides a higher extinctioncoefficient. Accordingly, the extinction coefficient of Al₂O₃high-refractive-index layers has a large influence on the transmittanceof the antireflection film. Also, as the geometrical thickness of theantireflection film increases, the antireflection film absorbs moreultraviolet light and, thus, the transmittance is reduced. Hence, thegeometrical thickness of the antireflection film also has a largeinfluence on the transmittance of the antireflection film.

The antireflection film of the present invention comprises alternatelydeposited high-refractive-index layers containing Al₂O₃ or the like andlow-refractive-index layers containing SiO₂ or the like. In theantireflection film, materials of the layers are selected so that thehigh-refractive-index layers and the low-refractive-index layers haveoptimum refractive indexes and extinction coefficients, for an ArFexcimer laser beam. In addition, the total geometrical thickness of thehigh-refractive-index layers, whose extinction coefficient is large, isset in a predetermined range. Consequently, the geometrical thickness ofthe resulting antireflection film can be reduced to increase thetransmittance. Thus, the antireflection film has satisfactoryantireflection characteristics for vacuum ultraviolet light.

If the geometrical thickness of the low-refractive-index layers, whoseextinction coefficient is small, is reduced, antireflectioncharacteristics are negatively affected. On the other hand,high-refractive-index layers whose geometrical thickness is reduced tosome extent maintain their antireflection characteristics and canincrease only their transmittance. Consequently, the geometricalthickness of the resulting antireflection film can be reduced toincrease the transmittance. Thus, the antireflection film hassatisfactory antireflection characteristics and suitably functions forvacuum ultraviolet light.

First Embodiment

A first embodiment will now be described with reference to the drawings.FIG. 1 is a sectional view of an optical element according to the firstembodiment, having an antireflection film on a parallel plate substrate.The antireflection film has a design center wavelength λ_(o) in therange of 180 to 220 nm, and advantageously functions particularly for anArF excimer laser beam having a wavelength of 193 nm. The substrate maybe a lens or a diffraction grating instead of the parallel plate.

Reference numerals 1 and 2 in FIG. 1 designate the substrate formed ofquartz and the antireflection film, respectively. The antireflectionfilm 2 is composed of six layers 2-1 to 2-6 counted from the substrateside. In the antireflection film 2, a high-refractive-index layer ispaired with a low-refractive-index layer, and a plurality of thesespairs are layered. In this instance, the uppermost layer (surface layer)must be a low-refractive-index layer. If the uppermost layer is not alower-refractive-index layer, the antireflection characteristics arenegatively affected. The layers are, therefore, deposited on thesubstrate, starting from the high-refractive-index layer. Hence, layers2-1, 2-3, and 2-5 are high-refractive-index layers containing Al₂O₃, andlayers 2-2, 2-4, and 2-6 are low-refractive-index layers containingSiO₂. The low-refractive-index layers 2-2, 2-4, and 2-6 have arefractive index ns and the high-refractive-index layers 2-1, 2-3, and2-5 have a refractive index na, for light having a wavelength in therange of 180 to 220 nm. The refractive indexes ns and na satisfy thefollowing expressions:1.50≦ns≦1.60, and1.74≦na≦1.77.

Also, the extinction coefficient ks of the low-refractive-index layers2-2, 2-4, and 2-6 and the extinction coefficient ka of thehigh-refractive-index layers 2-1, 2-3, and 2-5 for light having awavelength in the range of 180 to 220 nm satisfy the followingexpressions:0<ks≦1×10⁻⁵; and1×10⁻⁵ ≦ka≦1×10⁻³.

The respective optical thicknesses d1 to d6 of layers 2-1 to 2-6 satisfythe following expressions:0.02λ_(o) ≦d 1≦0.10λ_(o),1.00λ_(o) ≦d 2≦2.00λ_(o),0.22λ_(o) ≦d 3≦0.30λ_(o),0.22λ_(o) ≦d 4≦0.30λ_(o),0.22λ_(o) ≦d 5≦0.30λ_(o), and0.22λ_(o) ≦d 6≦0.30λ_(o).

The total optical thickness Dh1 (d1+d3+d5) of the high-refractive-indexlayers 2-1, 2-3, and 2-5 satisfy the expression 0.50λ_(o)≦Dh1≦0.60λ_(o).Since the optical thickness is defined by the product of the refractiveindex and the geometrical thickness, the total geometrical thickness Dhof high-refractive-index layers 2-1, 2-3, and 2-5 is determined to be inthe range of about 55 to about 65 nm from the expression Dh1=na×Dh.

An example according to the first embodiment will now be described.

EXAMPLE 1

The optical element of Example 1 uses a synthetic quartz as thesubstrate. The antireflection film was deposited on the surface of thesynthetic quartz substrate by sputtering. The antireflection film wascomposed of six layers. The layers at the first, third, and fifthpositions from the substrate side were high-refractive-index layers andthe layers at the second, fourth, and sixth positions werelow-refractive-index layers. The high-refractive-index layers containedAl₂O₃ and had a refractive index of 1.763 and an extinction coefficientof 3×10⁻⁴, at a wavelength of 193 nm. The low-refractive-index layerscontained SiO₂ and had a refractive index of 1.560 and an extinctioncoefficient of 0, at the same wavelength. Table 1 shows the structure ofthe antireflection film. In this instance, the total optical thicknessof the Al₂O₃ high-refractive-index layers, that is, of the first, third,and fifth layers, was 0.555λ_(o) and the geometrical thickness was about61 nm.

TABLE 1 Optical Material thickness Incident Air medium Sixth layer SiO₂0.254λ₀ Fifth layer Al₂O₃ 0.246λ₀ Fourth layer SiO₂ 0.263λ₀ Third layerAl₂O₃ 0.243λ₀ Second layer SiO₂ 1.608λ₀ First layer Al₂O₃ 0.066λ₀Emitting Quartz medium substrate

The reflection characteristics of the optical element having theantireflection film shown in Table 1 were measured by spectrophotometry.The results are shown in FIG. 2. The horizontal axis and the verticalaxis represent the wavelength and the reflectance, respectively. FIG. 2shows that the reflectance for ultraviolet light having a wavelength of193 nm, which is the design center wavelength λ_(o) of theantireflection film, is 0.1% or less. Thus, the antireflection filmexhibited highly effective antireflection characteristics. In general,an antireflection film exhibiting a reflectance of 0.2% or less hasexcellent antireflection characteristics.

The absorptance of the antireflection film was measured byspectrophotometry for ultraviolet light having a wavelength of 193 nm,which is the design center wavelength λ_(o). An absorptance of 0.14% wasobtained. In general, since an antireflection film exhibiting anabsorptance of 0.2% or less has excellent optical characteristics, theoptical element of Example 1 has excellent optical characteristics.

EXAMPLE 2

The optical element of Example 2 uses a synthetic quartz as thesubstrate. An antireflection film was deposited on the surface of thesynthetic quartz substrate by sputtering. The antireflection film wascomposed of six layers. The layers at the first, third, and fifthpositions from the substrate side are high-refractive-index layers. Thelayers at the second, fourth, and sixth positions arelow-refractive-index layers. The high-refractive-index layers containedAl₂O₃ and had a refractive index of 1.763 and an extinction coefficientof 3×10⁻⁴, at a wavelength of 193 nm. The low-refractive-index layerscontained SiO₂ and had a refractive index of 1.560 and an extinctioncoefficient of 0, at the same wavelength. Table 2 shows the structure ofthe antireflection film. In this instance, the total optical thicknessof the Al₂O₃ high-refractive-index layers, that is, of the first, third,and fifth layers, was 0.585λ_(o) and the geometrical thickness was about64 nm.

TABLE 2 Optical Material thickness Incident Air medium Sixth layer SiO₂0.254λ₀ Fifth layer Al₂O₃ 0.253λ₀ Fourth layer SiO₂ 0.257λ₀ Third layerAl₂O₃ 0.253λ₀ Second layer SiO₂ 1.103λ₀ First layer Al₂O₃ 0.079λ₀Emitting Quartz medium substrate

The reflection properties of the optical element having theantireflection film shown in Table 2 were measured by spectrophotometry.The results are shown in FIG. 3. The horizontal axis and the verticalaxis represent the wavelength and the reflectance, respectively. FIG. 3shows that the reflectance for ultraviolet light having a wavelength of193 nm, which is the design center wavelength λ_(o) of theantireflection film, is 0.2% or less. Thus, the antireflection filmexhibited highly effective antireflection characteristics.

The absorptance of the antireflection film was measured byspectrophotometry for ultraviolet light having a wavelength of 193 nm,which is the design center wavelength λ_(o), and an absorptance of 0.15%was obtained. Thus, the optical element of Example 2 exhibited excellentoptical characteristics.

Second Embodiment

A second embodiment will now be described with reference to thedrawings. FIG. 4 is a sectional view of an optical element according tothe second embodiment, having an antireflection film on a parallel platesubstrate. The antireflection film has a design center wavelength λ_(o)in the range of 180 to 220 nm, and advantageously functions particularlyfor an ArF excimer laser beam having a wavelength of 193 nm. Thesubstrate may be a lens or a diffraction grating instead of the parallelplate.

Reference numerals 10 and 3 in FIG. 4 designate a fluorite substrate andthe antireflection film, respectively. The antireflection film generallyincludes pairs of high-refractive-index and low-refractive index layers,as in the first embodiment shown in FIG. 1. The uppermost layer of theantireflection film is one of the low-refractive-index layers, becausean alternative configuration negatively impacts the antireflectioncharacteristics. Also, if the bottom layer in contact with the fluoritesubstrate is one of the high-refractive-index layer, light absorption isincreased in the interface and optical characteristics are negativelyaffected. Accordingly, an odd number of the layers are deposited on thesubstrate, starting from one of the low-refractive-index layers.

The antireflection film 3 is composed of seven layers 3-1 to 3-7,counted from the substrate side. Hence, layers 3-2, 3-4, and 3-6 arehigh-refractive-index layers containing Al₂O₃ or the like, and layers3-1, 3-3, 3-5, and 3-7 are low-refractive-index layers containing SiO₂or the like. The low-refractive-index layers 3-1, 3-3, 3-5, and 3-7 havea refractive index ns and the high-refractive-index layers 3-2, 3-4, 3-6have a refractive index na, for light having a wavelength in the rangeof 180 to 220 nm. The refractive indexes ns and na satisfy the followingexpressions:1.50≦ns≦1.60, and1.74≦na≦1.77.

The extinction coefficient ks of the low-refractive-index layers 3-1,3-3, 3-5, and 3-7 and the extinction coefficient of thehigh-refractive-index layers 3-2, 3-4, 3-6 satisfy the followingexpressions for light having a wavelength in the range of 180 to 220 nm:0<ks≦1×10⁻⁵, and1×10⁻⁵ ≦ka≦1×10⁻³.The respective optical thicknesses d1 to d7 of the layers 3-1 to 3-7satisfy the following expressions:0.00λ_(o) <d 1≦0.15λ_(o),0.02λ_(o) ≦d 2≦0.10λ_(o),1.20λ_(o) ≦d 3≦2.00λ_(o), 0.22λ_(o) ≦d 4≦0.30λ_(o),0.22λ_(o) ≦d 5≦0.30λ_(o),0.22λ_(o) ≦d 6≦0.30λ_(o), and0.22λ_(o) ≦d 7≦0.30λ_(o).

The total optical thickness Dh2 (d2+d4+d6) of the high-refractive-indexlayers 3-2, 3-4, and 3-6 satisfy the expression 0.50λ_(o)≦Dh2≦0.60λ_(o).Since the optical thickness is defined by the product of the refractiveindex and the geometrical thickness, the total geometrical thickness Dhof the high-refractive-index layers 3-2, 3-4, and 3-6 is determined tobe in the range of about 55 to about 65 nm from the expressionDh2=na×Dh.

An example according to the second embodiment will now be described.

EXAMPLE 3

The optical element of Example 3 uses fluorite as the substrate. Anantireflection film was deposited on the surface of the fluoritesubstrate by sputtering. The antireflection film was composed of sevenlayers. The layers at the first, third, fifth, and seventh positionsfrom the substrate side were low-refractive-index layers. The layers atthe second, fourth, and sixth positions were high-refractive-indexlayers. The low-refractive-index layers contained SiO₂ and had arefractive index of 1.560 and an extinction coefficient of 0, at awavelength of 193 nm. The high-refractive-index layers contained Al₂O₃and had a refractive index of 1.763 and an extinction coefficient of3×10⁻⁴, at the same wavelength. Table 3 shows the structure of theantireflection film. In this instance, the total optical thickness ofthe Al₂O₃ high-refractive-index layers, that is, of the second, fourth,and sixth layers, was 0.529λ_(o) and the geometrical thickness was about58 nm.

TABLE 3 Optical Material thickness Incident Air medium Seventh layerSiO₂ 0.254λ₀ Sixth layer Al₂O₃ 0.246λ₀ Fifth layer SiO₂ 0.263λ₀ Fourthlayer Al₂O₃ 0.242λ₀ Third layer SiO₂ 1.610λ₀ Second layer Al₂O₃ 0.041λ₀First layer SiO₂ 0.133λ₀ Emitting Fluorite medium substrate

The reflection properties of the optical element having theantireflection film shown in Table 3 were measured by spectrophotometry.The results are shown in FIG. 5. The horizontal axis and the verticalaxis represent the wavelength and the reflectance, respectively. FIG. 5shows that the reflectance for ultraviolet light having a wavelength of193 nm, which is the design center wavelength λ_(o) of theantireflection film, is 0.1% or less. Thus, the antireflection filmexhibited highly effective antireflection characteristics.

The absorptance of the antireflection film was measured byspectrophotometry for ultraviolet light having a wavelength of 193 nm,which is the design center wavelength λ_(o), and an absorptance of 0.14%was obtained. Thus, the optical element of Example 4 exhibited excellentoptical characteristics.

Third Embodiment

A third embodiment of the present invention will now be described withreference to the drawings. FIG. 6 is a sectional view of an opticalelement according to the third embodiment, having an antireflection filmon a parallel plate substrate. The antireflection film has a designcenter wavelength λ_(o) in the range of 180 to 220 nm, andadvantageously functions particularly for an ArF excimer laser beamhaving a wavelength of 193 nm. The substrate may be a lens or adiffraction grating instead of the parallel plate.

Reference numerals 20 and 4 in FIG. 6 designate the substrate formed ofquartz and the antireflection film, respectively. The antireflectionfilm 4 is composed of eight layers 4-1 to 4-8 counted from the substrateside. Hence, layers 4-1, 4-3, 4-5, and 4-7 are high-refractive-indexlayers containing Al₂O₃ or the like, and layers 4-2, 4-4, 4-6, and 4-8are low-refractive-index layers containing SiO₂ or the like. Thelow-refractive-index layers 4-2, 4-4, 4-6, and 6-8 have a refractiveindex ns and the high-refractive-index layers 4-1, 4-3, 4-5, and 4-7have a refractive index na, for light having a wavelength in the rangeof 180 to 220 nm. The refractive indexes ns and na satisfy the followingexpressions:1.50≦ns≦1.601.68≦na≦1.74

The low-refractive-index layers 4-2, 4-4, 4-6, and 4-8 have anextinction coefficient ks and the high-refractive-index layers 4-1, 4-3,4-5, and 4-7 have an extinction coefficient ka, for light having awavelength in the range of 180 to 220 nm. The extinction coefficient ksand ka satisfy the following expressions:0<ks≦1×10⁻⁵1×10⁻⁵ ≦ka≦1×10⁻³

The respective optical thicknesses d1 to d8 of the layers 4-1 to 4-8satisfy the following expressions:0.05λ_(o) ≦d 1≦0.20λ_(o),1.00λ_(o) ≦d 2≦1.50λ_(o),0.05λ_(o) ≦d 3≦0.15λ_(o),0.35λ_(o) ≦d 4≦0.45λ_(o),0.15λ_(o) ≦d 5≦0.25λ_(o), 0.27λ_(o) ≦d 6≦0.37λ_(o),0.16λ_(o) ≦d 7≦0.26λ_(o), and0.23λ_(o) ≦d 8≦0.33λ_(o).

The total optical thickness Dh3 (d1+d3+d5+d7) of thehigh-refractive-index layers 4-1, 4-3, 4-5, and 4-7 satisfy theexpression 0.59λ_(o)≦Dh3≦0.67λ_(o) Since the optical thickness isdefined by the product of the refractive index and the geometricalthickness, the total geometrical thickness Dh of thehigh-refractive-index layers 4-1, 4-3, 4-5, and 4-7 is determined to bein the range of about 65 to 75 nm from the expression Dh3=na×Dh.

An example according to the third embodiment will now be described.

EXAMPLE 4

The optical element of Example 4 uses a synthetic quartz as thesubstrate. An antireflection film was deposited on the surface of thesynthetic quartz substrate by sputtering. The antireflection film wascomposed of eight layers. The layers at the first, third, fifth, andseventh positions from the substrate side are high-refractive-indexlayers. The layers at the second, fourth, sixth, and eighth positionsare low-refractive-index layers. The high-refractive-index layerscontained Al₂O₃ and had a refractive index of 1.763 and an extinctioncoefficient of 3×10⁻⁴, at a wavelength of 193 nm. Thelow-refractive-index layers contained SiO₂ and had a refractive index of1.560 and an extinction coefficient of 0, at the same wavelength. Table4 shows the structure of the antireflection film. In this instance, thetotal optical thickness of the Al₂O₃ high-refractive-index layers, thatis, of the first, third, fifth, and seventh layers, was 0.659λ_(o) andthe geometrical thickness was about 72 nm.

TABLE 4 Optical Material thickness Incident Air medium Eighth layer SiO₂0.261λ₀ Seventh layer Al₂O₃ 0.218λ₀ Sixth layer SiO₂ 0.297λ₀ Fifth layerAl₂O₃ 0.200λ₀ Fourth layer SiO₂ 0.355λ₀ Third layer Al₂O₃ 0.114λ₀ Secondlayer SiO₂ 1.121λ₀ First layer Al₂O₃ 0.127λ₀ Emitting Quartz mediumsubstrate

The reflection properties of the optical element having theantireflection film shown in Table 4 were measured by spectrophotometry.The results are shown in FIG. 7. The horizontal axis and the verticalaxis represent the wavelength and the reflectance, respectively. FIG. 7shows that the reflectance for ultraviolet light having a wavelength of193 nm, which is the design center wavelength λ_(o) of theantireflection film, is 0.1% or less. Thus the antireflection filmexhibited highly effective antireflection characteristics.

The absorptance of the antireflection film was measured byspectrophotometry for ultraviolet light having a wavelength of 193 nm,which is the design center wavelength λ_(o), and an absorptance of 0.16%was obtained. Thus, the optical element of Example 4 exhibited excellentoptical characteristics.

Fourth Embodiment

A fourth embodiment of the present invention will now be described withreference to the drawings. FIG. 8 is a sectional view of an opticalelement according to the fourth embodiment, having an antireflectionfilm on a parallel plate substrate. The antireflection film has a designcenter wavelength λ_(o) in the range of 180 to 220 nm, andadvantageously functions particularly for an ArF excimer laser beamhaving a wavelength of 193 nm. The substrate may be a lens or adiffraction grating instead of the parallel plate.

Reference numerals 30 and 5 in FIG. 8 designate the substrate formed offluorite and the antireflection film, respectively. The antireflectionfilm 5 is composed of nine layers 5-1 to 5-9 counted from the substrateside. Hence, layers 5-1, 5-3, 5-5, 5-7, and 5-9 are low-refractive-indexlayers containing SiO₂ or the like, and layers 5-2, 5-4, 5-6, and 5-8are high-refractive-index layers containing Al₂O₃ or the like. Thelow-refractive-index layers 5-1, 5-3, 5-5, 5-7, and 5-9 have arefractive index ns and the high-refractive-index layers 5-2, 5-4, 5-6,and 5-8 have a refractive index na, for light having a wavelength in therange of 180 to 220 nm. The refractive indexes ns and na satisfy thefollowing expressions:1.50≦ns≦1.60, and0.68≦na≦1.74.

The low-refractive-index layers 5-1, 5-3, 5-5, 5-7, and 5-9 have anextinction coefficient ks and the high-refractive-index layers 5-2, 5-4,5-6, and 5-8 have an extinction coefficient ka, for light having awavelength in the range of 180 to 220 nm. The extinction coefficients ksand ka satisfy the following expressions:0<ks≦1×10⁻⁵,1×10⁻⁵ ≦ka≦1×10⁻³.

The respective optical thicknesses d1 to d9 of the layers 5-1 to 5-9satisfy the following expressions:0.00λ_(o) <d 1≦0.15λ_(o)0.05λ_(o) ≦d 2≦0.20λ_(o),0.04λ_(o) ≦d 3≦1.50λ_(o),0.05λ_(o) ≦d 4≦0.15λ_(o), 0.35λ_(o) ≦d 5≦0.45λ_(o),0.15λ_(o) ≦d 6≦0.25λ_(o),0.27λ_(o) ≦d 7≦0.37λ_(o),0.16λ_(o) ≦d 8≦0.26λ_(o), and0.23λ_(o) ≦d 9≦0.33λ_(o).

The total optical thickness Dh4 (d2+d4+d6+d8) of thehigh-refractive-index layers 5-2, 5-4, 5-6, and 5-8 satisfy theexpression 0.59λ_(o)≦Dh4≦0.67λ_(o). Since the optical thickness isdefined by the product of the refractive index and the geometricalthickness, the total geometrical thickness Dh of thehigh-refractive-index layers 5-2, 5-4, 5-6, and 5-8 is determined to bein the range of about 65 to 75 nm from the expression Dh4=na×Dh.

An example according to the fourth embodiment will now be described.

EXAMPLE 5

The optical element of Example 5 uses fluorite as the substrate. Anantireflection film was deposited on the surface of the fluoritesubstrate by sputtering. The antireflection film was composed of ninelayers. The layers at the first, third, fifth, seventh, and ninthpositions from the substrate side are low-refractive-index layers. Thelayers at the second, fourth, sixth, and eighth positions arehigh-refractive-index layers. The low-refractive-index layers containedSiO₂ and had a refractive index of 1.560 and an extinction coefficientof 0, at a wavelength of 193 nm. The high-refractive-index layerscontained Al₂O₃ and had a refractive index of 1.763 and an extinctioncoefficient of 3×10⁻⁴, at the same wavelength. Table 5 shows thestructure of the antireflection film. In this instance, the totaloptical thickness of the Al₂O₃ high-refractive-index layers, that is, ofthe second, fourth, sixth, and eighth layers, was 0.623λ_(o) and thegeometrical thickness was about 68 nm.

TABLE 5 Optical Material thickness Incident Air medium Ninth layer SiO₂0.261λ₀ Eighth layer Al₂O₃ 0.218λ₀ Seventh layer SiO₂ 0.298λ₀ Sixthlayer Al₂O₃ 0.200λ₀ Fifth layer SiO₂ 0.355λ₀ Fourth layer Al₂O₃ 0.115λ₀Third layer SiO₂ 1.135λ₀ Second layer Al₂O₃ 0.090λ₀ First layer SiO₂0.091λ₀ Emitting Fluorite medium substrate

The reflection properties of the optical element having theantireflection film shown in Table 5 were measured by spectrophotometry.The results are shown in FIG. 9. The horizontal axis and the verticalaxis represent the wavelength and the reflectance, respectively. FIG. 9shows that the reflectance for ultraviolet light having a wavelength of193 nm, which is the design center wavelength λ_(o) of theantireflection film, is 0.1% or less. Thus, the antireflection filmexhibited highly effective antireflection characteristics.

The absorptance of the antireflection film was measured byspectrophotometry for ultraviolet light having a wavelength of 193 nm,which is the design center wavelength λ_(o), and an absorptance of 0.15%was obtained. Thus, the optical element of Example 4 exhibited excellentoptical characteristics.

COMPARATIVE EXAMPLE

For comparison, the same measurement as in Examples 1 to 5 describedabove are performed on the antireflection film of Example 5 in JapanesePatent Laid Open No. 2000-206304. A synthetic quartz plate was used asthe substrate. An antireflection film was deposited on the surface ofthe synthetic quartz substrate by sputtering. The antireflection filmwas composed of eight layers. The layers at the first, third, fifth, andseventh positions from the substrate side are high-refractive-indexlayers and the layers at the second, fourth, sixth, and eighth positionsare low-refractive-index layers. The low-refractive-index layers wereformed of SiO₂ and had a refractive index of 1.560 at a wavelength of193 nm, and the high-refractive-index layers were formed of Al₂O₃ andhad a refractive index of 1.763 at the same wavelength. Table 6 showsthe structure of the antireflection film. The total optical thickness ofthe Al₂O₃ high-refractive-index layers, that is, of the first, third,fifth, and seventh layers, was 1,000λ_(o) and the total geometricalthickness was about 110 nm. This thickness is 1.69 to 2.00 times aslarge as the total geometrical thicknesses, 55 to 65 nm, of thehigh-refractive-index layers in above-described Examples 1 to 5.

TABLE 6 Optical Material thickness Incident Air medium Eighth layer SiO₂0.276λ₀ Seventh layer Al₂O₃ 0.252λ₀ Sixth layer SiO₂ 0.292λ₀ Fifth layerAl₂O₃ 0.238λ₀ Fourth layer SiO₂ 0.352λ₀ Third layer Al₂O₃ 0.453λ₀ Secondlayer SiO₂ 1.005λ₀ First layer Al₂O₃ 0.057λ₀ Emitting Quartz mediumsubstrate

The reflection properties of the optical element having theantireflection film shown in Table 6 were measured by spectrophotometry.The results are shown in FIG. 10. The horizontal axis and the verticalaxis represent the wavelength and the reflectance, respectively. FIG. 10shows that the reflectance for ultraviolet light having a wavelength of193 nm, which is the design center wavelength λ_(o) of theantireflection film, is 0.1% or less. Thus, the antireflection filmexhibited highly effective antireflection characteristics.

The spectral absorption of the optical element having the antireflectionfilm shown in Table 6 was also measured by spectrophotometry. Theresults are shown in FIG. 11. The horizontal axis and the vertical axisrepresent the wavelength and the absorptance, respectively. FIG. 11 alsoshows the spectral absorption in Example 1, for comparison. Theabsorptances in Example 1 and Comparative Example 1 are respectivelyabout 0.14% and 0.21% at a wavelength of 193 nm. In general, anantireflection film having a reflectance of 0.2% or less exhibitsexcellent antireflection characteristics. The antireflection film ofExample 1, therefore, has sufficiently low absorbance, and, accordingly,the resulting optical element has excellent optical characteristics. Incontrast, the antireflection film in the Comparative Example has anabsorbance higher than that in Example 1, and, accordingly, the opticalcharacteristics of the optical element in the Comparative Example areinferior.

The embodiments described above have illustrated the antireflectionfilms for an ArF excimer laser beam having a wavelength of 193 nm.However, the antireflection film of the present invention is not limitedto the uses associated with the ArF excimer laser beam. Thisantireflection film has similar antireflection characteristics of usedwith laser beams and other types of light having a wavelength in therange of about 180 to 220 nm. By disposing the antireflection film on asubstrate, the resulting optical element can exhibit excellent opticalcharacteristics for not only the ArF excimer laser beam, but also forother types of light.

The substrate used in the optical element is not limited to quartz andfluorite, and any material may be used as long as it has a transmittanceas high as that of quartz or fluorite.

A combination of a plurality of optical elements having theantireflection film may be applied to optical systems, such as thesystems for illuminating an exposure apparatus and for projectors.

While the present invention has been described with reference to whatare presently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation, so as to encompass all suchmodifications and equivalent structures and functions.

1. An optical element comprising: a quartz substrate; and anantireflection film comprising: four low-refractive-index layers each ofwhich has a refractive index ns satisfying the expression 1.50≦ns≦1.60and an extinction coefficient ks satisfying the expression 0<ks≦10⁻⁵,for ultraviolet light having a wavelength of 193 nm; and fourhigh-refractive-index layers each of which has a refractive index nasatisfying the expression 1.68≦na≦1.74 and an extinction coefficient kasatisfying the expression 1 10⁻⁵≦ka≦1×10⁻³, for ultraviolet light havinga wavelength of 193 nm, a total geometrical thickness of each of thefour high-refractive index layers being in a range of about 65 to about75 nm, wherein the four low-refractive-index layers and the fourhigh-refractive-index layers are alternately formed on the quartzsubstrate such that one of the high-refractive index layers is a mostproximal layer of the antireflection film to the quartz substrate,wherein the antireflection film has a design center wavelength λ_(o) ina range of 180 nm to 220 nm, and wherein respective optical thicknessesd1, d2, d3, d4, d5, d6, d7, and d8 of the high-refractive-index andlow-refractive-index layers at a first to eighth positions from asubstrate side satisfy the following expressions:0.05λ_(o) ≦d 1≦0.20λ_(o),1.00λ_(o) ≦d 2≦1.50λ_(o),0.05λ_(o) ≦d 3≦0.15λ_(o),0.35λ_(o) ≦d 4≦0.45λ_(o),0.15λ_(o) ≦d 5≦0.25λ_(o),0.27λ_(o) ≦d 6≦0.37λ_(o),0.16λ_(o) ≦d 7≦0.26λ_(o), and0.23λ_(o) ≦d 8≦0.33λ_(o).
 2. The optical element according to claim 1,wherein at least one of the high-refractive-index layers comprises Al₂O₃and at least one of the low-refractive-index layers comprises SiO₂.